Question:
Why does a three-phase motor in V/f characteristic operation possibly cause speed fluctuations that are not observed when the motor is operated in servo control at the servo inverter?
What is the purpose of the 'sway damping' function?
Answer:
Frequency inverter or servo inverter and motor control mode 'V/f characteristic operation':
When operating three-phase motors in controlled voltage-frequency characteristic operation (V/f), speed fluctuations (oscillations) can occur, especially in no-load operation.
These are excited by the residual ripple of the not ideally constant DC voltage in the DC link of the inverter.
The frequency inverter forms the voltages for the three motor phases from the DC voltage in the DC link using pulse width modulation (PWM).
The pulse pattern of the modulation is set in such a way that sinusoidal voltage curves result at constant DC link voltage and sinusoidal motor currents result accordingly.
Due to the existing residual ripple of the DC voltage, however, both variables deviate from the ideal sinusoidal curve, which can result in the aforementioned oscillations, especially at higher output frequencies.
Influencing factors favouring commuting:
- Idling operation
- higher-pole motors
- large motor power
Mode of operation of the pendulum damping:
In principle, a phase-shifted oscillation with the same frequency is superimposed on the oscillation of the motor current, so that the oscillation tendency is suppressed.
Further information on the mode of operation of the pendulum damping can be found in the documentation of the Lenze inverters.
The described oscillation behaviour cannot be observed with a servo inverter or the motor control type 'servo control'.
With a servo inverter, the motor current is set in a controlled manner in servo control by correspondingly adjusting the output motor voltage. The residual ripple of the DC voltage therefore has no significant effect.
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